The oilfield industry extracts petroleum oil and natural gas from wells drilled into the earth. In conventional oil and natural gas wells, a bore hole is drilled to the desired depth and the bore hole is lined with a casing that is essentially a round pipe installed in the bore hole for substantially the entire depth of the well. Sometimes the pipe is of the type usually called tubing but is used for the purpose of casing. The casing comprises many sections of pipe joined end to end, typically by threaded collars mating with threads on the sections of pipe, until the pipe reaches the desired depth, which is generally thousands of feet into the ground. The diameter of the casing depends on the drill bit used to drill the well, but is smaller than the diameter of the borehole. The outside diameter of the casing is typically 4½ to 20 inches, and the inside diameter is typically ½ inch less than the outside diameter. The casing is usually held in place in the bore hole by material, typically cement, injected between the casing and the side of the bore hole, and when this material solidifies the casing is said to be cemented in place.
In most cases, another pipe substantially smaller than the casing is inserted inside the casing, and held in a position approximately coaxial with the casing by devices of various types, which may be known as packers or as tubing hangers. This inner pipe is called production tubing, and is typically about 2 to 4 inches in outside diameter.
After the casing has been installed, completion of the well requires that the casing be perforated in the production region. The perforations allow oil and natural gas to move from the surrounding earth through the casing. The perforations penetrate both the casing and surrounding cement, and generally reach at least some small distance into the surrounding earth. It will be understood in the oilfield industry that the “earth” is usually rock but the term includes any solid material through which the well has been drilled. When the production tubing is in place, the oil and natural gas will flow continuously through the perforations and into the production tubing, which will have been manufactured with apertures to allow the inflow of oil and natural gas. The oil and natural gas will rise to the top of the production tubing, either by natural pressure on it arising in the earth near the bottom of the well, or by extractive technology such as pumping which is well known in the oilfield industry.
The usual method of perforating the casing uses multiple explosive charges fired simultaneously and configured to direct their explosive forces towards the casing. The device that carries the explosive charges is generally known as a perforating gun, or casing gun. In some types of wells not relevant to the present invention, the perforating gun may be attached to the production tubing for insertion into the well. In the methods of well construction relevant to the present invention, the perforating gun is lowered inside the casing on a wireline. A typical perforating gun is 3⅜ inches in diameter when intended for use in a casing that is 4 inches in inside diameter or larger. Perforating guns come in various lengths, generally between 3 feet and 22 feet, and a number of them may be joined according to the length of casing to be perforated.
Along with the perforating gun, the wireline may support other tools or devices intended for use in oil wells or natural gas wells, referred to here as “oilwell devices”. A common oilwell device is a device for determining the depth into the well reached by the perforating gun, so that the perforations will be made in the desired region of the casing where oil or natural gas is expected in the surrounding earth. One commonly used device for determining depth is a collar locator, which detects the magnetic effect of passing the collars that exist where each section of the casing joins another section. By counting the number of casing sections that have passed, each section being of known length, the depth of the collar locator in the well can be calculated. Alternatively, the depth reached by the perforating gun can be determined by observing the length of wireline that has been paid out as the perforating gun was lowered into the well hole. When the perforating gun is in the desired position in terms of depth downhole and orientation in the hole, the explosive elements are triggered to explode. A triggering electrical signal is often sent by an electrical conductor that forms part of the wireline or is associated with the wireline. In other cases, the perforating gun may be triggered mechanically, or by a sensor onboard the perforating gun.
It will be understood in the oilfield industry that “downhole” refers to the direction of the hole away from the point of origin of the drilling, whether the hole is vertical, slanted, curved, horizontal, or oriented in any manner. Similarly, “uphole” refers to the direction towards the point of origin of the drilling as seen from a position in the well.
In principle, the wireline could be attached directly to the perforating gun, with or without a collar locator, but generally the most uphole component in any perforating event, to which the wireline is attached, is an oilwell device with a fishing neck. A fishing neck, familiar in the wireline industry, is adapted to be seized by a grappling device from uphole if the fishing neck and whatever is attached to it have become stuck in the well, so that they can be extracted from the well. The present invention incorporates a fishing neck.
The explosive gas produced in the explosions of the perforating gun tends to lift both the perforating gun and the wireline to which it is attached, along with the fishing neck and any oilwell devices. The newly created perforations also allow an inrush of liberated oil or natural gas or both. In some cases, usually undesirably, the inrushing fluid is water. If the flow and pressure of the inrushing fluid are high, the inrushing fluid contributes to the lifting of the perforating gun and the wireline. For convenient exposition here, the term “pressurized fluid” will be used to denote any or all of gas from an explosion, natural gas, oil, water, and all gaseous or liquid substances that occur inside the casing in the region of the well at or downhole from the perforating gun, and that exert pressure uphole. Of course, the fluid pressure is exerted in all directions, but uphole is generally the only direction in which anything can move a significant distance.
A problem that arises from the lifting by the pressurized fluid of the perforating gun, along with the fishing neck and any collar locator or other oilwell device, is that the wireline goes slack and may become fouled or tangled. When the perforating gun is propelled uphole, it may overtake the wireline and force its way past the wireline and therefore jam or snag the wireline between the casing and any or all of the perforating gun, a fishing neck, a collar locator, and any other oilwell device. In many of such instances, the perforating gun and any oilwell device become jammed within the casing, and the wireline also becomes jammed within the casing, so that the well hole is obstructed. Then it is necessary to fish those obstructing objects out of the well hole, an operation of considerable difficulty and cost.
In prior art, attempts to prevent the perforating gun or other tool from being propelled uphole have used means for gripping the casing. Bowyer's U.S. Pat. No. 4,427,064 discloses a serrated surface that is radially expanded outward in response to upward movement. Hrupp's published Canadian Patent Application No. 2,227,354 discloses an device in which brake plugs are displaced radially outward by movement of a tapered piston that is driven by the explosive force. Such devices are more complex and more expensive that the present invention, and because they are highly mechanical they involve a risk of mechanical failure that is not possible with the present invention.
In order to obtain the most productive wells, high explosive pressure is desired in the perforating gun. With higher pressure, more or larger perforations can be obtained, and the holes punched into the rock can be deeper. With higher pressure, the edges of the perforations are cleaner and debris from the casing and rock tends to be cleared from the holes by being blown back into the casing, thereby eliminating a potential obstruction to the release of oil and natural gas. On the other hand, the higher the pressure, the greater the risk that the perforating gun and any oilwell device will be propelled uphole a considerable distance, so as to tangle the wireline and jam the assembly in the hole. Persons skilled in the art of wireline-controlled perforation attempt to find the ideal explosive charge to maximize perforations without excessive risk of propelling the perforating gun so far or so fast as to cause damage to the wireline and the equipment. The present invention allows larger explosions so that perforating will be more effective while the risk of jamming and a need for fishing will be less likely than with large explosions using the prior art. The larger explosions are especially useful when there is little or no fluid in the well before the perforating action, a circumstance known in the oilfield industry as underbalanced perforating.